Issues in Layout that do not fit into any other Layout component or which span multiple Layout components.

Bugs related to the top level presentation objects (pres shell, pres context, and document viewer), the frame constructor, and the base frame classes, as well as general issues with alignment and sizing, all belong here.

This is a recent regression of the full page zoom behavior, maybe from bug 389720, although this happens on all pages, not just framesets.
When you zoom in or out from 100%, the vertical and horizontal scrollbars are cut off or hidden entirely, depending on the zoom level, e.g. at 110%, I see only a very thin vertical scrollbar, maybe a fifth of its normal width, but still usable, and at 120%, the scrollbar is hidden entirely and I can't drag it.
The effect is visible at almost any zoom level except 50%, 100%, 150%, 200%.
I'm using the slider zoom extension from bug 389628, attachment 274906[details].

I can confirm it, and I can also confirm it is a somewhat recent regression.
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9a8pre) Gecko/2007080910 Minefield/3.0a8pre
By the way, I noticed resizing the window solves it, but also produces the reverse effect: when changing zoom, the display area may then become smaller than the available space (the unused space is left unpainted). Naturally, this can also be solved by resizing the window.